A procedure for modifying the surface composition of catalytically active metals with silicon-containing gaseous reactants has been developed. This new gas-solid reaction method is unique in that it can be used for the in situ synthesis of catalytically interesting materials, which cannot be done by conventional solid-solid reaction techniques. Using an oxygen-free silicon compound (e.g., hexamethyldisilazane, HMDS), the metals studied fall into two categories: those that involve reaction followed by diffusion, and those that exhibit surface reaction only. The first group, consisting of the metals Ni, V, Rh, Pt and Pd, formed thick (up to 0.6μ) Si diffusion layers, after reaction at 430 °C for a few hours, with either H2/HMDS or Ar/HMDS mixtures. Under the same conditions, the second group of metals, Fe and Co, showed thin (∼ 500 Å) overlayers containing silicon, but with no diffusion. The only nonmetal (graphite) studied so far showed no reaction, within the detectability limits of Auger spectroscopy. This observation shows that these reactions are catalytically induced by certain metal surfaces; in other words, they selectively take place on metals. These findings clearly have important implications for catalysis. For example, the metal surfaces of oxide-supported metal catalysts can, in principle, be selectively modified by gas-phase reactants. This treatment can readily be accomplished in situ in the catalytic reactor. The above reactions may well result in a new class of metallic catalysts, with one of the components being silicon. Furthermore, gas-phase compounds containing the elements aluminum, boron, and germanium are known to react with metals in an analogous manner, which further extends the range of possibilities for the synthesis of new catalytic materials.